1. Field of the Invention
This invention relates to a method and apparatus for anchoring a roof bolt in a bore hole of a rock formation and more particularly to an expansion shell assembly that is capable of being positioned at a preselected location along the length of a roof bolt to control the point of mechanically anchoring the roof bolt in the bore hole.
2. Description of the Prior Art
It is well known to reinforce and stabilize underground rock formations, such as a coal mine roof, a subway tunnel, or a similar subterranean structure or to strengthen a rock mass by the use of anchor bolts inserted within a bore hole drilled in the rock formation. The anchor bolts are tensioned during installation to reinforce the unsupported rock formation above the roof, for example, above the roof a mine passageway. Conventionally, a hole is drilled through the roof of the rock formation, and a rock bolt is inserted and anchored in the drilled hole either by engagement of a mechanical expansion shell with the wall of the rock formation around the hole or chemically anchoring the bolt by a multi component resin system or grout to the rock formation surrounding the hole.
With a mechanical expansion shell, the end of the bolt is threaded to receive the camming plug with an expansion shell positioned around the camming plug on the end of the bolt. Rotation of the bolt advances the camming plug downwardly relative to the shell to expand the fingers of the shell into gripping engagement with the rock formation. Thus, the expanded shell is anchored in the bore hole at the end of the bolt, and consequently the location of the expansion of the shell is fixed in the bore hole at the end of the bolt.
In a grout or resin anchoring system, the cartridges containing the separated chemical components are advanced into the bore hole ahead of a reinforcing rod. The cartridges are ruptured as the end of the rod passes through the cartridges annd is rotated. Rotation of the rod effects the mixing of the components. The mixture penetrates into the surrounding rock formation to adhesively unite the rock strata and anchor by bonding the rod to the rock formation surrounding the bore hole. The mixed resin or grout fills the annulus between the bore hole wall and the rod along a substantial length of the rod.
U.S. Pat. Nos. 3,108,443; 3,892,101; 3,940,941; 3,979,918; 4,051,683; 4,127,000; 4,129,007; 4,263,882; and 4,303,354 are examples of systems that use a grout or resin to anchor a roof bolt in a rock formation. U.S. Pat. Nos. 3,925,996 and 4,216,180 disclose multi component resin systems in which the resin mixture cures and begins to harden within a few seconds after mixing.
U.S. Pat. Nos. 3,877,235; 4,051,683; 4,023,373; and 4,275,975 disclose chemically anchored roof bolt systems that include an anchor portion which is inserted into the bore hole behind one or more resin cartridges and a lower portion which is connected to the anchor portion. With these devices, once the resin components are mixed and the mixture is cured to adhesively secure the anchor portion in the bore hole, application of a predetermined amount of torque to the bolt below the anchor portion releases the bolt for rotation relative to the anchor portion to draw a roof plate on the end of the bolt into compressive relation with the rock formation. In this manner the bolt is put under tension.
U.S. Pat. Nos. 4,129,007 and 4,132,080 disclose an anchor member that is internally threaded to receive the threaded end of the bolt. Both the anchor member and the bolt rotate initially as a unit to effect mixing of the resin. The bolt is not rotated during the time in which the mixed resin is permitted to set. After setting of the mixed resin, the bolt is rotated and the anchor member is restrained against rotation in the bore hole by setting of the resin. A preselected torque is applied to the bolt to tension the bolt.
U.S. Pat. Nos. 4,413,930 and 4,419,805 disclose method and apparatus for combining resin bonding and mechanical anchoring of bolt in a rock formation. With these devices a single bolt with a mechanical anchor positioned on the upper threaded end of the bolt is inserted in the bore hole behind the cartridge system. A roof plate is carried on the opposite headed end of the bolt for abutment against the rock formation surrounding the open end of the bore hole. The cartridge system is ruptured by the simultaneous upward thrust and rotation of the bolt to release the resin components for mixing. A stop device or other means associated with the expansion shell restrains expansion of the shell when the bolt is rotated in a preselected direction to mix the resin components. Rotation of the bolt continuoes without expansion of the shell for a predetermined period of time to permit the resin mixture to cure. As the resin mixture begins to harden, the shell expands into engagement with the wall of the bore hole and further rotation of the bolt exerts a tension on the bolt.
For both the mechanical anchor system and the combination resin and mechanical anchor system, it is the conventional practice to place the expansion shell assembly on the upper threaded end of the bolt. Normally the bolt is only threaded about four inches at the upper end portion to receive the expansion shell and, in some applications, at the lower end portion to receive a tightening nut. Consequently, the location in the bore hole where the shell member is expanded is limited to the area adjacent the end of the bore hole.
It is well known that the rock strata of a rock formation above an underground passageway can include a bedded formation of a variety of rock strata, such as shale, sandstone, mudstone, coal, and other rock laminations. The strata of a bedded formation vary in seam thickness and the seams may extend in a random path that deviates from a horizontal bedded formation. While certain strata such as slate may be very stable, the stable strata may be relatively thin, so that anchorage of the bolt end portion in the thin stable strata does not provide adequate support for the entire roof. With this type of roof it has been necessary in the past to drill bore holes though the strata above the stable slate to the next seam of stable slate or rock and anchor the end of the bolt by an expansion shell in the stable rock formation. In many instances this requires inordinately long bolt holes or bolt sections coupled to each other. The strata forming the roof immediately above the stable slate seam may be soft, and friable, and not provide adequate anchorage for an expansion shell. Thus, a mechanical anchor, which is tensioned during installation in a bed of soft, friable material is subject to slippage and loss in bolt tension as a result of deterioration of the surrounding material.
The use of a resin grouted bolt in the soft friable strata, provides a more suitable anchorage for the bolt end portion. The resin does not provide a tension on the bolt and as previously discussed, modifications must be made to the bolt system, such as a threaded external end on the bolt and a separate nut, to apply a tension to the bolt.
In an effort to maintain a larger area of contact between the expanded shell member with soft rock strata, special multiple anchors positioned in tandem on a bolt have been developed, as disclosed in U.S. Pat. No. 3,469,407. With this arrangement, two or more expansion shell assemblies are positioned on the threaded end portion of the bolt. The expansion shells are simultaneously expanded on the bolt end portion and anchored in the bolt hole. Accordingly, by increasing the threaded length of the bolt, the tandem arrangement of expansion shell assemblies can be spaced apart where, for example, the upper assembly is positioned in hard strata and the lower assembly is positioned in soft strata. Longitudinal spacing of the expansion shell assemblies on the threaded length of the bolt is controlled by a plastic tube, which is positioned between the assemblies.
U.S. Pat. No. 2,525,198 discloses an anchor bolt that includes an upper expansion shell assembly mounted on an upper threaded bolt, which is connected by a tubular member of a preselected length, to a lower threaded bolt, which is threaded into the opposite end of the tubular member. A lower expansion shell assembly is positioned on the lower threaded bolt. With this arrangement, the area of contact of the mechanical anchor with the rock formation is expanded. U.S. Pat. No. 3,303,736 discloses an expanion shell assembly adapted for positioning anywhere on the threaded portion of the bolt.
While it has been suggested by the prior art devices to utilize a combination of mechanical and chemical anchors to secure a bolt in a bore hole to overcome the problems associated with loss in bolt tension of a mechanically anchored bolt due to deterioation of the surrounding rock formation, the prior art devices limit the location of the expansion shell assembly to a specific threaded portion of the which is generally at the upper end of the bolt. The development of multiple anchors in tandem arrangement on a roof bolt also requires that a substantial length of the bolt be threaded or threaded extension members be utilized with an unthreaded bolt.
It also has been suggested by the prior art devices to locate an expansion shell assembly at a desired location along the threaded length of a bolt. This requires that the bolt be threaded a considerable length, if not along its entire length. Threaded a rock bolt along its entire length substantially increases the costs of a rock bolt and requires special handling to insure that the threads do not become damaged. Therefore, there is a need for roof bolt apparatus that includes an expansion shell assembly which is secured to a reinforcing rod or bolt at a preselected location along the length of the bolt, so as to permit adjustments in the location where the expanded shell member contacts the rock formation without requiring that the bolt be threaded at specific points along the length of the bolt or threaded along its entire length.